Which of the following processes makes direct use of oxygen?

Which one of the following processes in the bodies of living organisms is a digestive process?

The amount of which of the following components in the air does not change in the process of respiration?

In human body, the digestion of protein begins in which of the following organs?

Carbon monoxide poisoning affects mainly which one of the following?

Which among the following is the chief excretory product of human beings excreted via kidneys?